identical twins

mon·o·zy·got·ic twins

twins resulting from one zygote that at an early stage of development separated into independently growing cell aggregations giving rise to two individuals of the same sex and identical genetic constitution.

identical twins

identical twins

Monozygotic twins Twins resulting from the division of a single fertilized egg, which usually share a common chorion and placenta, although usually each may have a separate amnion. Cf Fraternal twins.

mon·o·zy·got·ic twins

(mon'ō-zī-got'ik twinz)

Twins resulting from a single zygote that at an early stage of development becomes separated into independently growing cell aggregations giving rise to two individuals of the same sex and identical genetic constitution. Synonym(s): enzygotic twins, identical twins.

twin

(twin)

TWINS: A. Monozygotic twins with one placenta, one chorion, and two amnions. B. Dizygotic twins with two placentas, two chorions, and two amnions

TWINS: A. Monozygotic twins with one placenta, one chorion, and two amnions. B. Dizygotic twins with two placentas, two chorions, and two amnions

Incidence

Per 1000 live births, incidence rates for American whites are 1:88; for American blacks, 1:70. Generally, the rates are higher in blacks and East Indians and lower in Northern Europeans.

Research on Twins

Identical and fraternal twins provide a unique resource for investigating the origin and natural history of various diseases and discovering the different rates of environmental and hereditary factors in causing physical and mental disorders. Esp. important are studies that follow the course of identical twins separated shortly after birth and who then grew up in different social, economic, educational, and environmental conditions. In other research, the second-born twin was found to be at increased risk for an unfavorable outcome (e.g., need for intubation and resuscitation, lower 5-min Apgar score), even when delivered by cesarean section.

identical twins

impacted twins

interlocked twins

Twins in which the neck of one becomes interlocked with the head of the other, making vaginal delivery impossible.

monozygotic twins

Twins that develop from a single fertilized ovum. Monozygotic twins have the same genetic makeup and, consequently, are of the same gender and strikingly resemble each other physically, physiologically, and mentally. They develop within a common chorionic sac and have a common placenta. Each usually develops its own amnion and umbilical cord. Such twins may result from development of two inner cell masses within a blastocyst, development of two embryonic axes on a single blastoderm, or the division of a single embryonic axis into two centers.

Here, the notion of doubling encapsulated by identical twins invokes the Freudian notion of the uncanny, 'that species of the frightening that goes back to what was once well known and had long been familiar'.

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